Short takeoff and landing, abbreviated STOL, refers to the ability of an aircraft to clear a 50-foot (15 meter) obstacle within 1500 feet (450 meters) of commencing takeoff, or in landing, to stop within 1500 feet after passing over a 50-foot obstacle. It is desirable under certain conditions for fixed-wing aircraft to be able to perform STOL operations at relatively low air speeds, for example twenty-thirty knots (37-55 kilometers/hr) indicated air speed. This requires either a high ratio of power to aircraft weight or high ratio of wing area to aircraft weight. Slats and flaps are the primary means for increasing the wing area of conventional aircraft. Typically, slats and flaps change the camber as well as increase the effective lift area of the wing. Increased wing area and changes in camber generally yield a greater lifting force on the wing, thereby reducing stall speed.
The benefits of reduced stall speed flight include short takeoff and landing roll. Additionally, reduced stall speed flight can prevent inadvertent stalls and permit high angles of climb, which may be useful at noise sensitive airports or at airports where obstacles encroach the glide path. Landing at slower speeds also decreases the wear and tear on brakes, landing gear assemblies, wing struts and tires.
There are some significant limitations on the use of slats and flaps. Slats and flaps should be retracted during cruising flight to reduce drag. Slats and flaps are complex flight control devices which may substantially increase the weight of an aircraft. Typically, the manufacturing and operating costs of aircraft utilizing slats and flaps are increased by the complexity of developing, manufacturing and maintaining the slat and/or flap assemblies. Also, retrofit installation on aircraft not originally equipped with slats and flaps involves major structural modifications.
Another method used to enable aircraft to takeoff and land at reduced stall speeds is through the development of lift-optimized wing designs. Increasing the camber of the airfoil of a wing can result in increased lift and decreased stall speeds. However, by using too much camber, drag penalties can become excessive for an aircraft operating at cruising speed. The same flight characteristics of the wing are present throughout all phases of flight. Consequently, conventional airfoil designs are optimized for all phases of flight (takeoff, cruise and landing) and must necessarily result in a compromise of various design factors which provide reasonable performance throughout all flight regimes. Conventional methods (installing slats/flaps and changing the airfoil) have been only marginally successful in reducing stall speed.
Instead of increasing the wing surface area, aircraft manufacturers sometimes increase the power available to an aircraft to reduce the takeoff roll. However, there are some limitations on the use of more powerful engines. First, an improved engine can be prohibitively expensive. Installation may not be feasible because of airframe limitations, maintenance can be more extensive, weight and balance factors may be affected and fuel consumption may be drastically increased. Moreover, the increased noise generated by a more powerful engine may not be tolerated at a noise sensitive airport.
Consequently, there is a continuing interest in providing an auxiliary wing structure that can be selectively deployed for enabling fixed wing aircraft to fly without stalling at slower speeds during takeoff and approach to landing, and that can be retracted during other phases of flight. This would permit the aircraft wings to be optimized for high speed cruise, with the auxiliary wing structure being deployed only during takeoff and landing, and retracted during high speed cruising flight.